The growing chain (GFP)mRNA tape and tRNAsEF-Tu / release factorA bacterial 70S ribosome, about 21 nm across (ruler: 1 tick = 1 nm), with the near half of the large subunit cut away; the mRNA tape is drawn about 3× stretched so its codons can be read. Each tooth is a base: purines (A, G) tall, pyrimidines (U, C) short, so only the right anticodon closes the gap. A tRNA arrives with EF-Tu, is tested in the A bay, and a near-cognate one is thrown back; the right one takes the chain from the P bay in a new peptide bond, then the tape and both tRNAs shift one codon and the empty one leaves from E. The tape is the real GFP gene, so the chain is GFP, 238 residues, ended by a release factor at UAA. A bacterial ribosome adds about 15–20 residues a second; shown 40× slower.